Transcriptional regulation of the type C retrovirus, feline leukemia virus (FeLV), depends upon the interaction of cellular encoded factors with specific sequences located in the U3 portion of the long terminal repeat (LTR). We have previously isolated a defective FeLV provirus from a T-cell lymphosarcoma which had transduced the proto-oncogene, c-myc. The transduced feline, v-myc, was shown to be identical to c-myc in nucleotide sequences, substantiating that amino acid substitution was not a requirement for transforming function. These data suggest that the level of transcripts emanating from the FeLV-myc provirus is a major determinant of transformation. To quantitate the promoter strength of the FeLV-myc LTR, the chloramphenicol acetyl transferase (CAT) assay was employed and data obtained were compared to the Garder Arnstein (GA) FeLV promoter. The results show that the FeLV-myc promoter is six times stronger than the GA FeLV promoter in T-cells, the tumor cell type from which FeLV-myc was cloned. In B-cells or fibroblasts, the FeLV-myc and GA FeLV promoters are equal in transcriptional strength. Two major differences are apparent in the FeLV-myc LTR compared with the GA LTR, a 44 base pair (bp) duplication and 7 nucleotide substitutions in U3. To assess the value of these alterations in conferring T-cell preference to the FeLV-myc promoter, in vitro recombinations were made between the FeLV-myc and GA LTR. Each construct was measured for T-cell preference using the CAT assay. The enhancer, a 44 bp duplication, accounts for 30% of this T-cell preference, while 5' and 3' nucleotide substitutions appear to be the major determinants.